Which packaging innovation in publishing is credited with making literature more accessible to the masses in the 20th century and was nicknamed 'the poor man's book'?

Book packaging has evolved dramatically over centuries, influencing how we discover and experience literature. From ancient scrolls to modern digital formats, the 'packaging' of written works reflects both technological advancement and artistic sensibility. This trivia question explores a significant innovation in book packaging that changed the literary marketplace and reading habits worldwide.

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Mass market paperbacks, pioneered by Penguin Books in the 1930s
Leather-bound collector's editions with gilt-edged pages
Dust jackets with embossed artwork, introduced in the 1920s
Hardcover books with illustrated slipcases